Fluoropolymer coated films useful for photovoltaic module

ABSTRACT

A photovoltaic module backsheet includes a polyester film having opposite first and second surfaces, and first and second fluoropolymer coatings on the first and second surfaces of the polyester film. The first and second fluoropolymer coatings each include fluoropolymer selected from homopolymers and copolymers of vinyl fluoride and homopolymers and copolymers of vinylidene fluoride polymer blended with compatible adhesive polymer. The compatible adhesive polymer includes a backbone that is compatible with the fluoropolymer and pendant functional groups. The pendant functional groups are selected from carboxylic acid, sulfonic acid, aziridine, anhydride, amine, isocyanate, melamine, epoxy, hydroxy and mixtures thereof. The surfaces of the polyester film include functional groups on each surface that interact with the pendant functional groups of the compatible adhesive polymer in the first and second fluoropolymer coatings to promote bonding of the first and second fluoropolymer coatings to the respective surfaces of the polyester film.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/642,183filed Dec. 20, 2006, now U.S. Pat. No. 7,553,540.

FIELD OF INVENTION

This invention relates to a fluoropolymer coated film which is useful asa backsheet in a photovoltaic module.

BACKGROUND OF THE INVENTION

Photovoltaic cells are used to produce electrical energy from sunlight,offering a green alternative to traditional methods of electricitygeneration. These solar cells are built from various semiconductorsystems which must be protected from environmental effects such asmoisture, oxygen, and UV light. The cells are usually jacketed on bothsides by encapsulating layers of glass and/or plastic films forming amultilayer structure known as a photovoltaic module. Fluoropolymer filmsare recognized as an important component in photovoltaic modules due totheir excellent strength, weather resistance, UV resistance, andmoisture barrier properties. Especially useful in these modules are filmcomposites of fluoropolymer film and polyester film which act as abacking sheet for the module. Such composites have traditionally beenproduced from preformed films of fluoropolymer, specifically polyvinylfluoride (PVF), adhered to polyester substrate film, specificallypolyethylene terephthalate. When fluoropolymer such as PVF is used as abacking sheet to the module, its properties significantly improve modulelife, allowing module warranties of up to 25 years. Fluoropolymerbacking sheets are frequently employed in the form of laminate withpolyethylene terephthalate (PET) films, typically with the PETsandwiched between two fluoropolymer films.

However, laminates of preformed fluoropolymer films on polymericsubstrates having a bond which will not delaminate after years ofoutdoor exposure are difficult to make. Prior art systems such as U.S.Pat. No. 3,133,854 to Simms, U.S. Pat. No. 5,139,878 to Kim et al., andU.S. Pat. No. 6,632,518 to Schmidt et al. describe primers and adhesivesfor preformed films that will produce durable laminate structures.However, these processes require the application of at least oneadhesive layer, or both a primer and adhesive layer, prior to the actuallamination step. The lamination step then requires the application ofheat and pressure to form the laminate. Therefore, prior art laminatesusing preformed fluoropolymer films are expensive to manufacture and/orrequire capital intensive equipment. Because preformed fluoropolymerfilms must have sufficient thickness to provide strength for handlingduring manufacture and subsequent processing, the resulting laminatesmay also incorporate thick expensive layers of fluoropolymer, i.e.,thicker than are necessary for an effective protective layer.

SUMMARY OF THE INVENTION

A photovoltaic module backsheet includes a polyester film havingopposite first and second surfaces, and first and second fluoropolymercoatings on the first and second surfaces of the polyester film,respectively. The first and second fluoropolymer coatings each includefluoropolymer selected from homopolymers and copolymers of vinylfluoride and homopolymers and copolymers of vinylidene fluoride polymerblended with compatible adhesive polymer. The compatible adhesivepolymer includes a backbone that is compatible with the fluoropolymerand pendant functional groups. The pendant functional groups areselected from carboxylic acid, sulfonic acid, aziridine, anhydride,amine, isocyanate, melamine, epoxy, hydroxy and mixtures thereof. Thefirst and second surfaces of the polyester film include functionalgroups on each surface that interact with the pendant functional groupsof the compatible adhesive polymer in the first and second fluoropolymercoatings to promote bonding of the first and second fluoropolymercoatings to the respective first and second surfaces of the polyesterfilm.

The invention provides a fluoropolymer coated polymer film with feweroverall processing steps than manufacturing laminates with preformedfluoropolymer films while providing strong adhesion to the substrate andgood durability to the fluoropolymer coated substrate. In addition,providing the fluoropolymer in the form of a coating enables thinnerfluoropolymer layers if desired to save cost. Employing fluoropolymercoatings also enables the incorporation of additives into thefluoropolymer layer tailored to the intended use of the fluoropolymercoated film, e.g., fillers which improve barrier properties.

DETAILED DESCRIPTION OF THE INVENTION

Fluoropolymers

Fluoropolymers useful in the fluoropolymer coated film in accordancewith the invention are selected from homopolymers and copolymers ofvinyl fluoride (VF) and homopolymers and copolymers of vinylidenefluoride (VF2) polymer. Preferably, the fluoropolymer is selected fromhomopolymers and copolymers of vinyl fluoride comprising at least 60mole % vinyl fluoride and homopolymers and copolymers of vinylidenefluoride comprising at least 60 mole % vinylidene fluoride. Morepreferably, the fluoropolymer is selected from homopolymers andcopolymers of vinyl fluoride comprising at least 80 mole % vinylfluoride and homopolymers and copolymers of vinylidene fluoridecomprising at least 80 mole % vinylidene fluoride. Blends of thefluoropolymers with nonfluoropolymers, e.g., acrylic polymers, may alsobe suitable for the practice of the invention. Homopolymer polyvinylfluoride (PVF) and homopolymer polyvinylidene fluoride (PVDF) are wellsuited for the practice of the invention.

For the practice of the invention with VF copolymers or VF2 copolymers,comonomers can be either fluorinated or nonfluorinated or mixturesthereof. By the term “copolymers” is meant copolymers of VF or VF2 withany number of additional fluorinated monomer units so as to formdipolymers, terpolymers, tetrapolymers, etc. If nonfluorinated monomersare used, the amount used should be limited so that the copolymerretains the desirable properties of the fluoropolymer, i.e., weatherresistance, solvent resistance, barrier properties, etc. Preferably,fluorinated comonomers are used including fluoroolefins, fluorinatedvinyl ethers, or fluorinated dioxoles. Examples of useful fluorinatedcomonomers include tetrafluoroethylene (TFE), hexafluoropropylene (HFP),chlorotrifluoroethylene (CTFE), trifluoroethylene,hexafluoroisobutylene, perfluorobutyl ethylene, perfluoro (propyl vinylether) (PPVE), perfluoro (ethyl vinyl ether) (PEVE), perfluoro (methylvinyl ether) (PMVE), perfluoro-2,2-dimethyl-1,3-dioxole (PDD) andperfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD) among many others.

Homopolymer PVDF coatings can be formed from a high molecular weightPVDF. Blends of PVDF and alkyl(meth)acrylates polymers can be used.Polymethyl methacrylate is particularly desirable. Typically, theseblends can comprise 50-70% by weight of PVDF and 30-50% by weight ofalkyl (meth)acrylate polymers, preferably, polymethyl methacrylate. Suchblends may contain compatibilizers and other additives to stabilize theblend. Such blends of polyvinylidene fluoride, or vinylidene fluoridecopolymer, and acrylic resin as the principal components are describedin U.S. Pat. Nos. 3,524,906; 4,931,324; and 5,707,697.

Homopolymer PVF coatings can be formed from a high molecular weight PVF.Suitable VF copolymers are taught by U.S. Pat. Nos. 6,242,547 and6,403,740 to Uschold.

Compatible Adhesive Polymers

The compatible adhesive polymers employed in the fluoropolymer coatedfilm according to the invention comprise functional groups selected fromcarboxylic acid, sulfonic acid, aziridine, amine, isocyanate, melamine,epoxy, hydroxy, anhydride and mixtures thereof. The compatible adhesivepolymer preferably has (1) a backbone composition that is compatiblewith the fluoropolymer in the composition and (2) pendant functionalitycapable of reacting with complementary functional groups on a substratefilm surface. The compatibility of the adhesive polymer backbone withthe fluoropolymer will vary but is sufficiently compatible so that theadhesive polymer can be introduced into the fluoropolymer in the desiredamount to secure the fluoropolymer coating to the substrate film. Ingeneral however, homo and copolymers derived largely from vinyl fluorideand vinylidene fluoride will show compatibility characteristics thatwill favor acrylic, urethane, aliphatic polyester, polyester urethane,acrylamide, urea and polycarbonate backbones having the functionalgroups described above.

The free radical addition polymers derived from acrylic and acrylamidemonomers are well suited to the introduction of pendant functionalgroups using the wealth of functional monomers available. Somerepresentatives include glycidyl acrylate and methacrylate for theintroduction of epoxy groups. These can then be converted into reactiveaminoalcohol groups by reaction of the epoxy functional acrylic withammonia or primary alkylamines. Carboxylic acid, isocyanate, hydroxy andanhydride functionalities are all available using acrylic/methacrylicacid, isocyanatoethyl methacrylate, hydroxyethyl methacrylate or maleicanhydride respectively. Numerous other functional monomers are availablefor functional group introduction as is well known in the art.

Preferred compatible adhesive polymers are amine functional polymers,more preferably, amine functional acrylic polymers.

Pigments and Fillers

If desired, various color, opacity and/or other property effects can beachieved by incorporating pigments and fillers into the fluoropolymercoating composition dispersion during manufacture. Pigments preferablyare used in amounts of about 1 to about 35 wt % based on fluoropolymersolids. Typical pigments that can be used include both clear pigments,such as inorganic siliceous pigments (silica pigments, for example) andconventional pigments. Conventional pigments that can be used includemetallic oxides such as titanium dioxide, and iron oxide; metalhydroxides; metal flakes, such as aluminum flake; chromates, such aslead chromate; sulfides; sulfates; carbonates; carbon black; silica;talc; china clay; phthalocyanine blues and greens, organo reds; organomaroons and other organic pigments and dyes. Preferred are pigments thatare stable at high temperatures during processing. It is also preferablethat the type and amount of pigment is selected to prevent anysignificant adverse affects on the desirable properties of fluoropolymercoating, e.g., weatherability.

Pigments can be formulated into a millbase by mixing the pigments with adispersing resin that may be the same as or compatible with thefluropolymer composition into which the pigment is to be incorporated.Pigment dispersions can be formed by conventional means, such as sandgrinding, ball milling, attritor grinding or two-roll milling. Otheradditives, while not generally needed or used, such as fiber glass andmineral fillers, anti-slip agents, plasticizers, nucleating agents, andthe like, can be incorporated.

UV Additives and Thermal Stabilizers

The fluoropolymer coating compositions may contain one or more lightstabilizers as additives. Light stabilizer additives include compoundsthat absorb ultraviolet radiation such as hydroxybenzophenones andhydroxybenzotriazoles. Other possible light stabilizer additives includehindered amine light stabilizers (HALS) and antioxidants. Thermalstabilizers can also be used if desired.

Barrier Particles

In a preferred embodiment, the fluoropolymer coating compositionincludes barrier particles. Preferably, the particles areplatelet-shaped particles. Such particles tend to align duringapplication of the coating and, since water, solvent and gases such asoxygen cannot pass readily through the particles themselves, amechanical barrier is formed in the resulting coating which reducespermeation of water, solvent and gases. In a photovoltaic module, forexample, the barrier particles substantially increase the moisturebarrier properties of the fluoropolymer and enhance the protectionprovided to the solar cells. If used, barrier particles are preferablypresent in the amount of about 0.5 to about 10% by weight based on thetotal dry weight of the fluoropolymer composition in the coating.

Examples of typical platelet shaped filler particles include mica, glassflake and stainless steel flake, and aluminum flake. The platelet shapedparticles are preferably mica particles, including mica particles coatedwith an oxide layer such as iron or titanium oxide. These particles havean average particle size of about 10 to 200 μm, preferably 20-100 μm,with no more than 50% of the particles of flake having average particlesize of more than about 300 μm. The mica particles coated with an oxidelayer are described in U.S. Pat. No. 3,087,827 (Klenke and Stratton);U.S. Pat. No. 3,087,828 (Linton); and U.S. Pat. No. 3,087,829 (Linton).The micas described in these patents are coated with oxides or hydrousoxides of titanium, zirconium, aluminum, zinc, antimony, tin, iron,copper, nickel, cobalt, chromium, or vanadium. Mixtures of coated micascan also be used.

Fluoropolymer Coating Composition Formulation

The fluoropolymer coating compositions may contain the fluoropolymereither in the form of a solution or dispersion of the fluoropolymer.Typical solutions or dispersions for the fluoropolymer are preparedusing solvents which have boiling points high enough to avoid bubbleformation during the film forming/drying process. For polymers indispersion form, a solvent which aids in coalescence of thefluoropolymer is preferable. The polymer concentration in thesesolutions or dispersions is adjusted to achieve a workable viscosity ofthe solution and will vary with the particular polymer, the othercomponents of the composition, and the process equipment and conditionsused. Preferably, for solutions the fluoropolymer is present in anamount of about 10 wt % to about 25 wt % based the total weight of thecomposition. For dispersions, the fluoropolymer is preferably present inan amount of about 25 wt % to about 50 wt % based the total weight ofthe composition.

The form of the polymer in the coating composition is dependent upon thetype of fluoropolymer and the solvent used. Homopolymer PVF is normallyin dispersion form. Homopolymer PVDF can be in dispersion or solutionform dependent upon the solvent selected. For example, homopolymer PVDFcan form stable solutions at room temperature in many polar organicsolvents such as ketones, esters and some ethers. Suitable examplesinclude acetone, methylethyl ketone (MEK) and tetrahydrofuran (THF).Depending upon comonomer content and the solvent selected, copolymers ofVF and VF2 may be used either in dispersion or solution form.

In one preferred form of the invention using homopolymer polyvinylfluoride (PVF), suitable coating formulations are prepared usingdispersions of the fluoropolymer. The nature and preparation ofdispersions are described in detail in U.S. Pat. Nos. 2,419,008;2,510,783; and 2,599,300. Preferred PVF dispersions are formed indimethyl acetamide, propylene carbonate, y-butyrolactone, N-methylpyrrolidone, and dimethylsulfoxide.

To prepare the fluoropolymer coating composition in dispersion form, thefluoropolymer and the compatible adhesive polymer and, optionally one ormore dispersants and/or pigments, are generally first milled together ina suitable solvent. Alternatively, the various components are milled orappropriately mixed separately. Components which are soluble in thesolvent such as the compatible adhesive polymer do not require milling.

A wide variety of mills can be used for the preparation of thedispersion. Typically, the mill employs a dense agitated grindingmedium, such as sand, steel shot, glass beads, ceramic shot, Zirconia,or pebbles, as in a ball mill, an ATTRITOR® available from UnionProcess, Akron, Ohio, or an agitated media mill such as a “Netzsch” millavailable from Netzsch, Inc., Exton, Pa. The dispersion is milled for atime sufficient to cause deagglomeration of the PVF. Typical residencetime of the dispersion in a Netzsch mill ranges from thirty seconds upto ten minutes.

The adhesive polymer is employed in the coating composition at a levelsufficient to provide the desired bonding to the polymeric substratefilm but below the level at which the desirable properties of thefluoropolymer would be significantly adversely affected. Preferably,coating composition contains about 1 to about 40 wt % adhesive polymer,more preferably about 1 to about 25 wt % adhesive polymer, and mostpreferably 1 to about 20 wt % adhesive polymer, based on the weight ofthe fluoropolymer.

Substrate Films and Primers

Polymeric substrate films used in this invention may be selected from awide number of polymers, with thermoplastics being preferred. Thepolymeric substrate film comprises functional groups on its surface thatinteract with the compatible adhesive polymer to promote bonding of thefluoropolymer coating to the substrate film. The polymeric substratefilm is preferably a polyester, and more preferably a polyester selectedfrom the group consisting of polyethylene terephthalate, polyethylenenaphthalate and a coextrudate of polyethylene terephthalate/polyethylenenaphthalate.

Fillers may also be included in the substrate film, where their presencemay improve the physical properties of the substrate, for example,higher modulus and tensile strength. They may also improve adhesion ofthe fluoropolymer to the substrate film. One exemplary filler is bariumsulfate, although others may also be used.

The surface of the polymeric substrate film which is to be coated maynaturally possess functional groups suitable for bonding as in hydroxyand/or carboxylic acid in a polyester film or amine and/or acidfunctionality in a polyamide film. Many polymeric substrate films mayneed or would further benefit from activation however, and this may beachieved by surface treatment. That is, the surface can be madereceptive by forming functional groups of carboxylic acid, sulfonicacid, aziridine, amine, isocyanate, melamine, epoxy, hydroxy, anhydrideand/or mixtures thereof on the surface. The activation can be achievedby chemical exposure such as to a gaseous Lewis acid such as BF₃ or tosulfuric acid or to hot sodium hydroxide. Preferably, the surface can beactivated by exposing one or both surfaces to an open flame whilecooling the opposite surface. Activation can also be achieved bysubjecting the film to a high frequency, spark discharge such as coronatreatment or atmospheric nitrogen plasma treatment.

In a preferred embodiment of this invention, a primer layer is depositedon the polymer substrate film and provides the functional groups thatinteract with the compatible adhesive polymer in the fluoropolymercoating composition to promote bonding of the fluoropolymer coating tothe substrate film. In one embodiment, the primer layer has a thicknessof about 20 to about 100 nm. Suitable primers may include polyamines,polyamides, acrylamide polymers (especially amorphous acrylamides),polyethyleneimines, ethylene copolymers or terpolymers, acid-modifiedpolyolefins (e.g. maleated polyolefins), acrylate or methacrylatepolymers (e.g., emulsion polymers), polyester (e.g., dispersions),polyurethanes (e.g., dispersions), epoxy polymers, epoxyacrylicoligomers, and mixtures thereof. An example of this is the introductionof amine functionality by the application of a polyethyleneimine primercoating. A second example is coextrusion of an acid or anhydridefunctional thermoplastic polymer, such as the polymer sold by the DuPontCompany under the trademark BYNEL®, with the base PET substrate film.When primers are used on, for example, PET substrate films which arestretched during manufacture, the primer can be applied either before orafter the substrate film has been stretched.

Coating Application

The fluoropolymer compositions for making the fluoropolymer coated filmin accordance with the present invention can be applied as a liquiddirectly to suitable polymeric substrate films by conventional coatingmeans with no need to form a preformed film. Techniques for producingsuch coatings include conventional methods of casting, dipping, sprayingand painting. When the fluoropolymer coating contains fluoropolymer indispersion form, it is typically applied by casting the dispersion ontothe substrate film, using conventional means, such as spray, roll,knife, curtain, gravure coaters, or any other method that permits theapplication of a uniform coating without streaks or other defects. Sprayand roller applications are the most convenient application methods. Thedry coating thickness of cast dispersion is preferably about 2.5 μm (0.1mil) and about 250 μm (10 mils), preferably between about 13 μm (0.5mil) to about 130 μm (5 mils).

After application, the wet solutions or dispersion are dried to removethe solvent and coalesced thermally if necessary to form thefluoropolymer coating on the substrate film. With some compositions inwhich the fluoropolymer is in solution form, the compositions can becoated onto substrate films and allowed to air dry at ambienttemperatures. Although not necessary to produce a coalesced film,heating is generally desirable to dry the coating more quickly. Dryingtemperature thus preferably in the range of about 25° C. (ambientconditions) to about 200° C. (oven temperature—the film temperature willbe lower). The temperature used should also be sufficient to promote theinteraction of the functional groups in the adhesive polymer with thefunctional groups of the polymeric substrate film to provide securebonding of the fluoropolymer coating to the substrate film. Thistemperature varies widely with the adhesive polymer employed and thefunctional groups of substrate film and can range from room temperatureto oven temperatures in excess of that required for the coalescence offluoropolymers in dispersion form as discussed below.

When the fluoropolymer in the composition is in dispersion form, it isnecessary for the solvent to be removed and also for the fluoropolymerto be heated to a sufficiently high temperature that the fluoropolymerparticles coalesce into a continuous film. Preferably, fluoropolymer inthe coating is heated to temperature of about 150° C. to about 250° C.The solvent used preferably aids in coalescence, i.e., enables a lowertemperature to be used for coalesce than would be necessary with nosolvent present. Thus, the conditions used to coalesce polymer will varywith the fluoropolymer used, the thickness of the cast dispersion andthe substrate film, and other operating conditions. For homopolymer PVFcoatings and residence times of about 1 to about 3 minutes, oventemperatures of about from 340° F. (171° C.) to about 480° F. (249° C.)can be used to coalesce the film, and temperatures of about 380° F.(193° C.) to about 450° F. (232° C.) have been found to be particularlysatisfactory. The oven air temperatures, of course, are notrepresentative of the temperatures reached by the fluoropolymer coatingwhich will be lower.

In a preferred form of the invention, the fluoropolymer coatingcomposition is applied to a substrate film. Preferably, the substratefilm includes a primer layer providing functional groups that interactwith said compatible adhesive polymer to promote bonding. Preferably,the substrate film is polyester such a polyethylene terephthalate,polyethylene napthalate or a coextrudate of polyethyleneterephthalate/polyethylene naphthalate. In another preferred form of theinvention, the fluoropolymer coating is applied to both surfaces of thesubstrate film. This can be performed simultaneously on both sides ofthe polymeric substrate film or alternately, the coated substrate filmcan be dried, turned to the uncoated side and resubmitted to the samecoating head to apply coating to the opposite side of the film toachieve coating on both sides of the film.

Photovoltaic Modules

Fluoropolymer coated films made in accordance with the invention areespecially useful in photovoltaic modules. A typical construction for aphotovoltaic module includes a thick layer of glass as a glazingmaterial. The glass protects solar cells comprising crystalline siliconwafers and wires which are embedded in a moisture resisting plasticsealing compound such as cross-linked ethylene vinyl acetate.Alternatively thin film solar cells can be applied from varioussemiconductor materials, such as CIGS (copper-indium-gallium-selenide),CTS (cadmium-tellurium-sulfide), a-Si (amorphous silicon) and others ona carrier sheet which is also jacketed on both sides with encapsulantmaterials. Adhered to the encapsulant is a backsheet. Fluoropolymercoated films made in accordance with the invention are useful for suchbacksheets. The fluoropolymer coating comprises fluoropolymer selectedfrom homopolymers and copolymers of vinyl fluoride and homopolymers andcopolymers of vinylidene fluoride polymer blended with compatibleadhesive polymer containing functional groups selected from carboxylicacid, sulfonic acid, aziridine, anhydride, amine, isocyanate, melamine,epoxy, hydroxy, anhydride and mixtures thereof. The polymeric substratefilm comprises functional groups on its surface that interact with thecompatible adhesive polymer to promote bonding of the fluoropolymercoating to the substrate film. The polymeric substrate film ispreferably a polyester, and more preferably a polyester selected fromthe group consisting of polyethylene terephthalate, polyethylenenaphthalate and a coextrudate of polyethylene terephthalate/polyethylenenaphthalate. Polyester provides electrical insulation and moisturebarrier properties, and is an economical component of the back sheet.Preferably both surfaces of the polymeric substrate film is coated withfluoropolymer creating a sandwich of polyester between two layers ofcoating of fluoropolymer. Fluoropolymer films provide excellentstrength, weather resistance, UV resistance, and moisture barrierproperties to the backsheet.

EXAMPLES Test Methods

180 Degree Peel Strength

Peel strengths are measured using a Model 4201 Instron at 2″/min,recording the peak value and averaging 3 samples (ASTM D1876-01 T-PeelTest). If samples easily peeled by hand during the peel initiation stepa value of 0 was recorded.

Humidity Cabinet Peel Test

After removal from the humidity cabinet the samples are scored with arazor knife and a straight edge to produce ¼″ wide strips. The 1″overhang is used as a handle and this tab is pulled at roughly a 180degree angle with slow even tension until either the film breaks or apeel results. Film breaks and peels at either the EVA/glass orEVA/fluoropolymer coating interfaces are considered passing results.Peels between the fluoropolymer coating and the PET substrate areconsidered failures.

Cross-Hatch Adhesion

After removal from the humidity cabinet the samples are scored with arazor knife, aided by a stainless steel template, to make 11 parallelcuts about 3/32 inch (2.4 mm) apart through the film to the glasssurface. This procedure is repeated at right angles to the first cuts toproduce a grid of 100 squares. A strip of transparent tape (3M Brand No.467 PSA tape), 0.75 by 2.16 inch (1.9 by 5.5 cm), is pressed firmly overthe scribed area with the tape oriented in a parallel direction to thescribed lines. The tape is then pulled off at a 90° angle rapidly butwithout jerking. Any failure between the fluoropolymer coating and thePET substrate is considered a failure.

Water Vapor Transmission Rate (WVTR)

Water Vapor Transmission Rate is measured using a Mocon Permatron-W® 700Instrument at 37.8° C. and a permiant relative humidity of 100%.

Examples 1-5

Examples 1 to 5 illustrate bonding PVF based coatings containing anamine functional acrylic adhesive or an epoxy functional acrylicadhesive being used to coat polyethylene terephthalate (PET) filmspreviously primed with either polyethyleneimine (PEI) or ethyleneacrylic acid copolymer (EAA). Results show that in general more adhesivepolymer is better. Results indicate that no significant bonding isachieved in the absence of the adhesive polymer.

PVF Based Coating Formulations

TABLE 1 Fluoropolymer Coating Formulations Amount Formu- PVF AdhesiveAdhesive Wt % Adhesive lation Dispersion* Polymer** Polymer based on PVFA 20.0 Amine functional 2.8 10 acrylic B 20.0 Amine functional 5.6 20acrylic C 20.0 Epoxy functional 2.2 10 acrylic D 20.0 Epoxy functional4.4 20 acrylic E 20.0 None 0 0 *PVF (available from DuPont) previouslydispersed into propylene carbonate (available from Huntsman) at 42%solids **amine functional acrylic is methylmethacrylate/2-hydroxy-3-aminopropyl methacrylate 97.8/2.2 at 30% solidsin 2-propanol/toluene 55/45. **Epoxy functional acrylic is methylmethacrylate/glycidyl methacrylate 98/2 at 38% solids in2-propanol/toluene 55/45.PET+Primer Substrates

TABLE 2 Substrates with PEI Coatings PET thickness Coating ThicknessSubstrate (microns) PEI (nm) Sample 1 100 Lupasol ® P 20 Sample 2 100Lupasol ® SKA 20 Sample 3 100 Lupasol ® SKA 40

TABLE 3 Substrates with EAA Coatings Substrate PET thickness (microns)Coating Thickness (nm) Sample 4 100 40 Sample 5 100 80Procedure

-   -   1. Ingredients of Table 1 are combined and agitated 15 minutes        on a paint shaker to mix.    -   2. Draw downs of the mixtures in Table 1 are prepared on each of        the 4 mil PET webs in Tables 2 and 3 using a 12 mil draw down        knife to produce approximately 1.2 mil dry fluoropolymer        coatings.    -   3. A heavy bead of pure PVF dispersion (42% in propylene        carbonate) is applied along 1 edge of the wet draw down to help        facilitate peel testing.    -   4. The coated webs are clamped into metal frames and placed into        preheated ovens at either 200° C. or 220° C. for 3 minutes.    -   5. The coalesced and dried films are removed from the ovens and        allowed to cool.    -   6. One inch strips are cut perpendicular to the heavy PVF bead.    -   7. A scalpel is used to initiate peeling at the PVF bead.    -   8. Peel strengths are measured using a Model 4201 Instron at        2″/min, recording the peak value and averaging 3 samples (ASTM        D1876-01 T-Peel Test). If samples easily peeled by hand during        the peel initiation step a value of 0 was recorded.

TABLE 4 Peel Testing Results 220° C. Fluoro- 200° C. Bake Temp Bake Temppolymer Peel Strength Peel Strength Example Substrate Formulation(KG/in) (KG/in) 1A Sample 1 A 0 0 1B Sample 1 B 0 0.76 1C Sample 1 C 0 01D Sample 1 D 1.18 1.14* 1E Sample 1 E 0 0 2A Sample 2 A 0.78 0.57 2BSample 2 B 1.2 1.47 2C Sample 2 C 0 0 2D Sample 2 D 1.46 1.28* 2E Sample2 E 0 0 3A Sample 3 A 0 0.59 3B Sample 3 B 1.36 1.26 3C Sample 3 C 0 03D Sample 3 D 1.48 1.42* 3E Sample 3 E 0 0 4A Sample 4 A 1.39 1.44 4BSample 4 B 1.53 1.33 4C Sample 4 C 0 1.07 4D Sample 4 D 1.29 1.35* 4ESample 4 E 0 0 5A Sample 5 A 1.15 1.35 5B Sample 5 B 1.44 1.59 5C Sample5 C 1.2 1.13 5D Sample 5 D 0 1.38* 5E Sample 5 E 0 0 *denotesformulation/substrate/bake temperature combinations that survive the1000 hr 85° C./85% humidity cabinet test (only selected samples aretested).Humidity Cabinet TestingSample Preparation

Fluoropolymer coated PET substrates are prepared in the same manner asthose of examples 1 through 5 with the exception that the edge bead ofstep 3 was omitted. After removing the samples from the oven andallowing them to cool, the fluoropolymer coating side is corona treatedusing a hand held lab corona treater. The corona treated sample was thenlaminated to a glass panel (4″×8″×⅛″) using the following procedure.

The following sandwich is placed on a vacuum plate with a 1″fluoropolymer coated PET overhang:

-   -   --------- fluoropolymer coated PET film with fluoropolymer        coating face down    -   - - - - reactive EVA film    -   ------- glass panel        -   1. The sandwich is centered and covered with a silicone            rubber pad        -   2. A metal frame is placed on the rubber pad and vacuum            applied for 20 minutes        -   3. The sandwich+vacuum plate is placed into a room            temperature oven and heating begun to a target of 150° C.        -   4. The sample is held at 150° C. for 20 minutes        -   5. The sample is removed from the oven, vacuum released and            allowed to cool        -   6. The resulting laminates are placed into a paint sample            rack in a humidity cabinet controlled at 85° C./85%            humidity.        -   7. Samples are exposed to the humidity cabinet for 1000            hours and then examined for adhesion using both peel testing            and a cross hatch tape test.

Examples 6-9

Examples 6-9 illustrate PVF coating compositions containing an aminefunction acrylic adhesive being used to coat PET films primed withpolyallylamine primer.

Example 6

Example 6 illustrates the application of a white PVF coating to bothsides of unpigmented and pigmented PET film primed with a polyallylamineprimer.

A white PVF coating formulation is prepared from the formula in thefollowing Table 5. The thermal stabilizer solution listed in Table 5 isprepared from 4.0 wt % Irganox® 1035 (Ciba), 1 wt % Weston® THOP(Crompton) and 95 wt % propylene carbonate. The amine function acrylicpolymer solution listed in Table 5 is prepared by post reacting methylmethacrylate/glycidyl methacrylate (98/2) with ammonia to convert theglycidyl groups into 2-hydroxy-3-aminopropyl groups to produce a primaryamine functional acrylic polymer in a solvent of toluene andisopropanol.

TABLE 5 Ingredient Wet wt. Dry wt. Wt % based on PVF PVF dispersion (40%in PC) 58.55 23.42 N.A. White pigment dispersion 23.55 10.65 45.5Thermal stabilizer sol'n 2.9 0.15 0.66 Amine functional acrylic sol'n14.43 4.32 18.4

Using a doctor blade, the white PVF coating formulation is coated on a 2mil unpigmented PET film primed with a polyallylamine primer and onbarium sulfate pigmented 3.8 mil wide PET film primed with apolyallylamine primer. The coatings are baked in a preheated oven for 10min at 200° C. for 10 minutes. The cooled coated films are then coatedon the opposite side with the same PVF formulation and are baked 15 minat 200° C. Table 6 shows the range of resultant dry film thicknesses forcoatings on pigmented and unpigmented PET films:

TABLE 6 film thickness (mils) sample PET Pigmented? PET 1st PVF Coating2nd PVF Coating 1 Yes 3.8 1 0.7 2 Yes 3.8 1.2 1.5 3 No 2 1 1 4 No 2 1.21.3 5 No 2 1 1 6 No 2 1.2 1.3 7 Yes 3.8 0.7 1 8 Yes 3.8 1.2 1

Example 7

Example 7 illustrates the application of a PVF coating containingpearlescent barrier particles of mica to both sides of unpigmented andpigmented PET film primed with a polyallylamine primer.

A pearlescent PVF coating formulation is prepared from the formula inthe following Table 7:

TABLE 7 Wt % PVF Dispersion 43.03 Mearlin ® Sparkle 139P (Englehard)10.63 Themal Stabilizer Solution 2.13 Amine functional acrylic polymersolution 10.63 Propylene Carbonate 33.58

Using a doctor blade, the pearlescent PVF coating formulation is coatedon a 2 mil wide, unpigmented PET film primed with a polyallylamineprimer and on a 3.8 mil wide, barium sulfate pigmented PET film primedwith a polyallylamine primer. These coatings are baked in a preheatedoven for 10 min at 200° C. for 10 minutes. The cooled coated films arethen coated on the opposite side with the same PVF formulation and baked15 min at 200° C. Table 8 shows the range of resultant dry coatingthicknesses:

TABLE 8 film thickness (mils) sample PET Pigmented? PET 1st PVF Coating2nd PVF Coating 1 Yes 3.8 0.8 0.5 2 Yes 3.8 0.7 1 3 Yes 3.8 1 1.7 4 Yes3.8 0.7 1.6 5 Yes 3.8 0.95 1.05 6 Yes 3.8 0.7 1 7 Yes 3.8 1.2 1 8 Yes3.8 1.2 1.1 9 No 2 1 1 10 No 2 0.9 1 11 No 2 1 1 12 No 2 1.3 1.45 13 No2 1.3 1.5

Example 8

In Example 8, the vapor barrier properties of two side coated PET filmsof Example 6 (sample 7) and Example 7 (sample 2) having white pigmentedand mica pigmented fluoropolymer coatings, respectively, are evaluatedby measuring the Water Vapor Transmission Rate (WVTR). Table 9 shows theimprovement of vapor barrier properties when using barrier particlessuch as mica, as indicated by a lower WVTR value.

TABLE 9 PVF/PET/PVF Side 1 Side 2 thicknesses WVTR Pigment Pigment(mils) g/m²/day white white 0.7/3.8/1.0 5.53 pearl pearl 0.7/3.8/1.02.052

Example 9

In Example 9, the shrinkage properties of white two side coated PETsamples made according to Example 6 (sample 1) are compared with samplesof PET films with conventional preformed PVF films laminated to bothsides. The former contained a filler (barium sulfate) in the PETsubstrate, while the latter did not. Table 10 shows the measuredshrinkage data at various temperatures.

TABLE 10 ° F. Two side Coated Film Two side Laminated Film % Change inMachine Direction Dimension as a function of Temperature 25 0.000 0.00050 0.000 0.000 100 0.000 −0.084 150 −0.083 −0.669 200 −0.083 −3.177 %Change in Tranverse Direction Dimension as a function of Temperature 250.000 0.000 50 0.000 −0.083 100 0.084 −0.083 150 0.000 −0.166 200 0.084−2.912 Note: Two side Coated film layer thicknesses (mils) 1/3.8/0.7 Twoside laminated film layer thicknesses (mils) 1.5/3/1.5

Example 10

Example 10 illustrates the application of a white pigmented and anunpigmented polyvinylidene (PVDF) coating to one side of an unpigmentedand a pigmented PET film primed with a polyallylamine primer.

The unpigmented PVDF coating formulation is prepared from the followingcomponents shown in Table 11:

TABLE 11 Material Source Wt % Polyvinylidene Fluoride Aldrich 21.03Amine functional acrylic polymer solution (1) DuPont 32.9 PropyleneCarbonate Huntsman 46.07 (1) Methyl methacrylate/glycidyl methacrylate(98/2) post reacted with ammonia to convert the glycidyl groups into2-hydroxy-3-aminopropyl groups to produce a primary amine functionalacrylic polymer in a solvent of toluene and isopropanol.

The mixture is dispersed on a paint shaker in the presence of 3 mm glassbeads for 10 minutes.

The white pigmented coating formulation is prepared from the followingcomponents shown in Table 12:

TABLE 12 Material Wt % Clear PVDF Formulation 78.32 White dispersion21.68

Using a 7 mil doctor blade, both the clear PVDF formulation and thewhite PVDF formulation are coated on unpigmented 2 mil PET film andpigmented 3.8 mil PET film primed with a polyallylamine primer using a7-mil doctor blade. A comparison 3.8 mil unprimed PET is similarlycoated. The coated films were baked in a preheated 400° F. convectionoven for 5 minutes. After cooling, a scalpel was used to attempt toremove the PVF films from PET films. Both the pigmented and unpigmentedfilms could be peeled from unprimed PET but could not be peeled fromprimed PET.

1. A photovoltaic module backsheet comprising: a polyester film havingopposite first and second surfaces, first and second fluoropolymercoatings on said first and second surfaces of said polyester film,respectively, said first and second fluoropolymer coatings eachcomprising fluoropolymer selected from homopolymers and copolymers ofvinyl fluoride and homopolymers and copolymers of vinylidene fluoridepolymer blended with compatible adhesive polymer comprising a backbonethat is compatible with said fluoropolymer and pendant functionalgroups, said pendant functional groups selected from carboxylic acid,sulfonic acid, aziridine, anhydride, amine, isocyanate, melamine, epoxy,hydroxy and mixtures thereof; wherein said first and second surfaces ofsaid polyester film comprise functional groups on each surface thatinteract with said pendant functional groups of the compatible adhesivepolymer in said first and second fluoropolymer coatings to promotebonding of said first and second fluoropolymer coatings to saidrespective first and second surfaces of said polyester film, and whereinsaid first and second fluoropolymer coatings are first and secondsurface layers of the photovoltaic module backsheet.
 2. The photovoltaicmodule backsheet of claim 1, wherein the backbone of the compatibleadhesive polymer in said first and second fluoropolymer coatings isselected from acrylic, urethane, aliphatic polyester, polyesterurethane, acrylamide, urea and polycarbonate backbones.
 3. Thephotovoltaic module backsheet of claim 1 wherein said compatibleadhesive polymer is a functional acrylic polymer.
 4. The photovoltaicmodule backsheet of claim 1 wherein said compatible adhesion-promotingpolymer is an amine functional acrylic polymer.
 5. The photovoltaicmodule backsheet of claim 1 wherein said functional groups on the firstand second surfaces of said polyester film are selected from carboxylicacid, sulfonic acid, aziridine, amine, isocyanate, melamine, epoxy,hydroxy, anhydride and mixtures thereof.
 6. The photovoltaic modulebacksheet of claim 1 comprising first and second primers on saidrespective first and second surfaces of said polyester film, said firstand second primers providing said functional groups on the first andsecond surfaces of said polyester film that interact with said adhesivepolymer to promote bonding of said first and second fluoropolymercoatings to said respective first and second surfaces of said polyesterfilm.
 7. The photovoltaic module backsheet of claim 6 wherein said firstand second primers comprise a polymer selected frompolyallylamine/melamine polymer, polyethylene imine polymer, ethyleneacrylic acid copolymer, and acid modified polyolefin.
 8. Thephotovoltaic module backsheet of claim 6 wherein at least one of saidfirst and second primers is an acid or anhydride functionalthermoplastic polymer that is coextruded with said polyester film. 9.The photovoltaic module of claim 1 wherein said first and secondfluoropolymer coatings each have a thickness of about 0.1 to about 2.0mils.
 10. The photovoltaic module backsheet of claim 1 wherein saidfirst and second fluoropolymer coatings each comprise platelet-shapedbarrier particles having an average particle size of 10 to 200 microns.11. The photovoltaic module backsheet of claim 10 wherein said first andsecond fluoropolymer coatings each comprise about 0.5 to about 10 weight% of platelet-shaped barrier particles based on total dry weight of thefluoropolymer in each fluoropolymer coating.
 12. The photovoltaic modulebacksheet of claim 1 wherein said fluoropolymer of said first and secondfluoropolymer coatings is selected from homopolymers and copolymers ofvinyl fluoride comprising at least 60 mole % vinyl fluoride andhomopolymers and copolymers of vinylidene fluoride comprising at least60 mole % vinylidene fluoride.
 13. The photovoltaic module backsheet ofclaim 1 wherein said fluoropolymer of said first and secondfluoropolymer coatings comprises about 1 to about 40 weight % of saidcompatible adhesive polymer based on fluoropolymer solids content. 14.The photovoltaic module backsheet of claim 1 wherein said polyester filmhas a thickness of about 0.5 to about 10 mils.
 15. A photovoltaic modulecomprising: photovoltaic cells; an encapsulating layer on saidphotovoltaic cells; a backsheet adhered to the encapsulating layer, thebacksheet comprising a polyester film having opposite first and secondsurfaces, and first and second fluoropolymer coatings on said first andsecond surfaces of said polyester film, respectively, said first andsecond fluoropolymer coatings each comprising fluoropolymer selectedfrom homopolymers and copolymers of vinyl fluoride and homopolymers andcopolymers of vinylidene fluoride polymer blended with compatibleadhesive polymer comprising a backbone that is compatible with saidfluoropolymer and pendant functional groups, said pendant functionalgroups selected from carboxylic acid, sulfonic acid, aziridine,anhydride, amine, isocyanate, melamine, epoxy, hydroxy and mixturesthereof, wherein said first and second surfaces of said polyester filmcomprise functional groups on each surface that interact with saidpendant functional groups of the compatible adhesive polymer in saidfirst and second fluoropolymer coatings to promote bonding of said firstand second fluoropolymer coatings to said respective first and secondsurfaces of said polyester film.
 16. The photovoltaic module of claim15, wherein the backbone of the compatible adhesive polymer in saidfirst and second fluoropolymer coatings is selected from acrylic,urethane, aliphatic polyester, polyester urethane, acrylamide, urea andpolycarbonate backbones.
 17. The photovoltaic module of claim 15,wherein said functional groups on the first and second surfaces of saidpolyester film are selected from carboxylic acid, sulfonic acid,aziridine, amine, isocyanate, melamine, epoxy, hydroxy, anhydride andmixtures thereof.
 18. The photovoltaic module of claim 15, wherein saidfirst and second fluoropolymer coatings each have a thickness of about0.1 to about 2.0 mils, and wherein said first and second fluoropolymercoatings each comprise platelet-shaped barrier particles having anaverage particle size of 10 to 200 microns.
 19. The photovoltaic moduleof claim 18, wherein said first and second fluoropolymer coatings eachcomprise about 0.5 to about 10 weight % of platelet-shaped barrierparticles based on total dry weight of the fluoropolymer in eachfluoropolymer coating.